1. Field of the Invention
The present invention pertains to rigid cellular foams characterized by carbodiimide linkages. More particularly, the present invention pertains to urethane-modified rigid cellular foams characterized by carbodiimide linkages. Even more particularly, the present invention concerns urethane-modified rigid cellular foams characterized by carbodiimide linkages having skins of improved flame retardancy, as well as reduced friability and which can be prepared from distilled toluene diisocyanate.
2. Prior Art
There has been described heretofore the preparation of rigid cellular foams characterized by carbodiimide linkages. See, inter alia, U.S. Pat. Nos. 3,645,923; 3,717,596; 3,723,366 and 3,772,217. Generally, these rigid cellular foams are prepared by the catalytic condensation of an organic polyisocyanate in the presence of suitable catalytic systems which promote both carbodiimide and isocyanurate formation. The resulting foams, thus, contain carbodiimide linkages, isocyanurate groups, as well as, some free isocyanate. To enhance the properties of the foams, the prior art, also, teaches the inclusion into the foam formulation of various active hydrogen-containing compounds. Yet, the prior art foams were found to be deficient with respect to friability and the inability to utilize distilled toluene diisocyanate.
To alleviate these problems, there has been developed urethane-modified carbodiimide foams from crude methylene diphenyl diisocyanate, and the like. See, inter alia, U.S. Pat. Nos. 3,645,923; 3,723,336; 3,748,239; 3,766,103; 3,732,189; 3,806,475; 3,824,240; 3,824,239; etc. More recently, there has been developed urethane-modified carbodiimide foams prepared from distilled toluene diisocyanate as well as acid-modified distilled toluene diisocyanate. According to this development, a quasi-prepolymer, a two-step or a one-step process is employed wherein the isocyanate is reacted in the presence of a highly reactive polyol to cause urethane formation, thereby alleviating the problem of friability. These developmens are more particularly described in copending U.S. Pat. application Ser. No. 511,111, filed Oct. 2, 1974 and entitled "Urethane-Modified Carbodiimide-Isocyanurate Foams Prepared From TDI-Rich Isocyanate" and copending U.S. Pat. application Ser. No. 511,112, filed Oct. 2, 1974, and entitled "Carbodiimide-Isocyanurate Foams Prepared From Acid-Modified Toluene Diisocyanate".
Although the urethane-modified foams exhibit improved properties over previous foams, not so-modified, it has been found that the skins thereof have a high urethane content, with respect to carbodiimide and isocyanurate groups, as compared to the core, and, thus, exhibit a lower flame retardancy than the core. The present invention is, therefore, directed to a solution for upgrading the flame retardancy of the skin of the urethane-modified carbodiimide foams.